Disclosed herein is site-directed mutagenesis of a cloned phyA gene employed to replaced cysteine residues involved in disulfide bridge formation with another amino acid. Also disclosed herein is an isolated mutant phytase comprising an amino acid sequence having at least 96 percent sequence identity to SEQ. ID. NO: 6 and containing a double-substitution amino acid residue substitution of residue 31 and residue 40 of SEQ. ID. NO: 6, wherein said isolated mutant phytase has phytase activity.